Chromatography is a widely used technique for separating mixtures. Generally it involves two relatively moving phases, e.g. a stationary phase (solid) and a mobile phase (fluid). "Normal phase" chromatography uses a polar stationary phase and a non-polar mobile phase. In "reverse phase" chromatography the stationary phase is non-polar. A common stationary phase is silica; particulate amorphous silica. Reverse phase chromatography conventionally uses silica which has been modified by covalent attachment of long hydrocarbon chains to give a non-polar surface. This "reverse phase" silica is protected from hydrolysis in aqueous mobile phases because of the absence of polar interactions at its surface; the consequent use of aqueous mobile phases permits a variety of new separations. This is important in the medical field where many compounds are at least to some extent soluble in water, and includes analytical chromatography techniques such as thin layer chromatography (TLC).
However, reverse phase chromatography is often avoided for reasons of expense. Reverse phase silica media are up to ten times as expensive as their normal phase counterparts, because of the multi-step reactions with various silanes involved in manufacture.